Multilayer Films

ABSTRACT

There is provided a multilayer film comprising a starch layer and at least one other layer. The multilayer film has excellent barrier properties. The starch layer comprises a modified starch having a degree of substitution less than 1.5. Suitable other layers include polyolefins. The multilayer film finds use in packaging, particularly in packaging foodstuffs.

FIELD OF THE INVENTION

The present invention relates to multilayer films having a starch basedlayer and to methods of their preparation. The films find particular,although not exclusive, use in the packaging of foodstuffs.

BACKGROUND TO THE INVENTION

Multilayer films find widespread use as a packaging material in the foodindustry. In such application not only does the packaging material haveto contain the foodstuff, it must also preserve the product. Foods areby their nature perishable and the properties of the packaging materialwill greatly impact the useful shelf life of a given foodstuff. Chemicalchanges, such as oxidation, and microbiological growth can beaccelerated in the presence of oxygen. Accordingly, controlling theoxygen content in a package or the rate of oxygen ingress into a packageis often one of the most critical attributes of barrier packaging.

Multilayer barriers are usually constructed with a polar polymer as aninternal layer which is covered with apolar polymer. The former acts asa gas barrier, and the latter as a hydrophobic skin having a low watervapour transmission rate to prevent fast water absorption in theinternal layer. Polyolefin skin layers covering a polar polymer layerare commonly utilised, for example, a polyethylene skin covering apolyethylene-vinyl alcohol copolymer (PE-EVOH).

EVOH copolymers show good oxygen barrier properties at low humidity,typically in the range of 0 to 60%. However, their gas barrier propertydeteriorates dramatically under high humidity conditions when thehumidity is in the range of 75 to 90%. In fact, due to the polar natureof EVOH, such films generally exhibit poor moisture barrier. Therefore,EVOH is typically laminated with polyolefins on both sides to providebarrier properties for practical packaging applications in order toprotect the EVOH from humidity effects. However, over time sufficientmoisture may permeate the polyolefin hydrophobic skin such that theoxygen barrier properties of the EVOH layer will be compromised.

EVOH materials also require the use of adhesion promoters and/ortie-layer resins in order for them to bond adequately to polyolefinsubstrates. Without such tie resins, EVOH materials tend to peel offeasily from the polyolefin substrate resulting in loss of barrierproperties and poor appearance.

A further disadvantage of EVOH is that it is relatively expensive.Additionally, from a renewable standpoint, EVOH is fully derived fromfossil fuels.

It would be desirable to provide a multilayer film that has excellentbarrier properties, particularly at high humidity and which utilisesinexpensive, renewable materials.

SUMMARY OF THE INVENTION

According to a first aspect there is provided a multilayer filmcomprising:

(a) at least one starch layer comprising a modified starch; and

(b) at least one other layer having a water vapour permeabilitycoefficient less than 1 g·mm/m²·24 hr·atm measured at 38° C. and 90%relative humidity; and wherein the total thickness of the at least onestarch layer is greater than 20% of the total thickness of themultilayer film and wherein the modified starch has a degree ofsubstitution less than 1.5.

Preferably the water vapour permeability coefficient of the at least oneother layer is less than 0.5 g·mm/m²·24 hr·atm measured at 38° C. and90% relative humidity, more preferably less than 0.2 g·mm/m²·24 hr·atmmeasured at 38° C. and 90% relative humidity.

Preferably the total thickness of the at least one starch layer isgreater than 30% of the total thickness of the multilayer film, morepreferably greater than 40% of the total thickness of the multilayerfilm, even more preferably greater than 50% of the total thickness ofthe multilayer film. In some embodiments the total thickness of the atleast one starch layer is greater than 60% of the total thickness of themultilayer film.

The multilayer film has a low oxygen permeability coefficient (OPC). Insome embodiments, the multilayer film has an OPC less than 0.6 cm³mm/m²·24 h·atm at 50% relative humidity (RH). Preferably the multilayerfilm has an OPC of less than 0.3 cm³ mm/m²·24 h·atm at 50% RH and morepreferably an OPC of less than 0.2 cm³ mm/m²·24 h·atm at 50% RH. Mostpreferably, the multilayer film has an OPC of less than 0.1 cm³ mm/m²·24h·atm at 50% RH and particularly preferably the multilayer film has anOPC of less than 0.05 cm³ mm/m²·24 h·atm at 50% RH.

In some embodiments, the multilayer film has an OPC less than 1.2 cm³mm/m²·24 h·atm at 75% relative humidity (RH). Preferably the multilayerfilm has an OPC of less than 0.6 cm³ mm/m²·24 h·atm at 75% RH and morepreferably an OPC of less than 0.2 cm³ mm/m²·24 h·atm at 75% RH. Mostpreferably, the multilayer film has an OPC of less than 0.1 cm³ mm/m²·24h·atm at 75% RH and particulary preferably the multilayer film has anOPC of less than 0.05 cm³ mm/m²·24 h·atm at 75% RH.

In some embodiments, the OPC remains below 0.05 cm³ mm/m²·24 h·atm at50% RH, for extended periods of time. Preferably the OPC remains below0.05 cm³ mm/m²·24 h·atm at 50% RH for at least ten days, more preferablythe OPC remains below 0.05 cm³ mm/m²·24 h·atm at 50% RH for 20 days,most preferably the OPC remains below 0.05 cm³ mm/m²·24 h·atm at 50% RHfor thirty days. In a particularly preferred embodiment the OPC remainsbelow 0.05 cm³ mm/m²·24 h·atm at 50% RH for thirty days.

Accordingly, the multilayer films have an enhanced performance inrespect of oxygen barrier properties over long periods of time. Whilenot wishing to be bound by theory it is considered that the highmoisture capacity of the at least one starch layer acts to extend thelifetime of the oxygen barrier effect, even when moisture levels withinthe at least one starch layer are relatively high.

Such longevity in respect of gas barrier performance is clearlydesirable in extending the shelf life of packaged perishable foodstuffs.

Advantageously, from a renewable perspective the multilayer filmcontains a high proportion of biodegradable starch.

The thickness of the multilayer film and of each layer within themultilayer film may vary depending on the exact nature of the end useapplication.

Preferably, the total thickness of the multilayer film is between 10 and1000 microns. In one embodiment the total thickness of the multilayerfilm is between 10 and 100 microns, more preferably between 20 and 80microns. In another embodiment the total thickness of the multilayerfilm is between 100 and 1000 microns more preferably between 200 and 800microns.

In some embodiments, the total thickness of the at least one starchlayer is between 5 and 600 microns. In one embodiment, the totalthickness of the at least one starch layer is between 5 and 50 microns,preferably between 10 and 40 microns. In other embodiments, the totalthickness of the at least one starch layer is between 100 and 600microns, preferably between 150 and 450 microns.

In some embodiments, the total thickness of the at least one other layeris between 5 and 400 microns. In one embodiment, the total thickness ofthe at least one other layer is between 5 and 25 microns preferablybetween 10 and 20 microns. In another embodiment, the total thickness ofthe at least one other layer is between 30 and 400 microns, preferablybetween 30 and 300 microns.

In a preferred embodiment the at least one starch layer has a totalthickness between 100 and 600 micron and the at least one other layerhas a total thickness between 10 and 400 micron. In another preferredembodiment the at least one starch layer has a total thickness between100 and 400 micron and the at least one other layer has a totalthickness between 40 and 250 micron.

In another preferred embodiment the at least one starch layer has atotal thickness between 10 and 60 micron and the at least one otherlayer has a total thickness between 5 and 40 micron.

Other Layer

The other layer(s) may be chosen to impart certain physical andaesthetic properties to finished multilayer film. These properties mayinclude, for example, antifog, strength, heat sealability, colour, orclarity. In some embodiments particularly preferred other layers arethose having a low water vapour transmission rate.

In some embodiments the at least one other layer comprises a polyolefin,polyethylene terephthalate, nylon, polyvinylchloride and polyvinylidenedichloride or mixtures thereof. In one embodiment each of the otherlayers may comprise a mixture of components. In other embodiments one ormore of the other layers may consist of multiple layers of differentmaterials. In further embodiments each of the other layers may comprisedifferent materials.

In some embodiments suitable polyolefins for the preparation of thepolyolefin film layer are selected from the group consisting of ethylenehomopolymers, propylene homopolymers, interpolymers of ethylene andpropylene and interpolymers of ethylene or propylene with one or moreC₄-C₁₀ α-olefins, cyclic olefin polymers and copolymers, biaxiallyorientated polypropylene, and mixtures thereof.

In some embodiments suitable polyolefins are selected from copolymers ofethylene or propylene and one or more α-olefins. Both high densitypolyethylenes and linear low density polyethylenes may be preferablyutilised.

Suitable linear low density polyethylenes (LLDPE) include copolymers ofethylene and α-olefins (about 5 to about 15 wt. %). Alpha-olefinsinclude 1-butene, 1-hexene, 1-octene, and the like, and mixturesthereof. The density of LLDPE is within the range of about 0.865 toabout 0.925 g/cm³

Suitable high density polyethylenes (HDPE) include ethylene homopolymersand copolymers of ethylene and α-olefins (about 0.1 to about 10 wt. %).Suitable alpha-olefins include 1-butene, 1-hexene, 1-octene, and thelike, and mixtures thereof. The density of HDPE is preferably from about0.940 to about 0.970 g/cm³.

Suitable cyclic olefin polymers and copolymers include polymers ofnorbornene or tetracyclododecene and copolymers of norbornene ortetracyclododecene with one or more α-olefins. Examples are cyclicolefin polymers are Topas (Ticona) and Apel (Mitsui).

In some embodiments blends of polyolefins and other polymers may beadvantageously employed. Cast polypropylene (cPP) or biaxially orientedpolypropylene (BOPP) may be chosen for improved strength and low WVTR.Polyethylene terephthalate (PET) may be chosen for strength andshrinkability.

In other embodiments modified polyolefins, such as grafted polyolefins,may be utilised. A preferred grafted polyolefin is a maleic anhydridegrafted polyolefin.

Starch Layer

The multilayer film comprises at least one starch layer comprising amodified starch, wherein the modified starch has a degree ofsubstitution less than 1.5. The degree of substitution defines theaverage number of substituents per anhydroglucose unit. Accordingly, bydefinition, the maximum possible degree of substitution of starch is3.0.

In one embodiment the at least one starch layer comprises a high amylosestarch. Preferably, the amount of high amylose starch is between 5 and80% by weight based on the total weight of the starch layer.

In another embodiment the modified starch is chemically modified so asto replace hydroxyl functionality with functionality selected from thegroup consisting of ethers and esters and mixtures thereof. Preferredesters comprise heptanoate or lower homologues. Particularly preferredesters include acetate.

In a further embodiment the modified starch is modified to include ahydroxyalkyl C₂₋₆ group or modified by reaction with an anhydride of acarboxylic acid. Preferably the modified starch is modified to include ahydroxy C₂₋₄ group. More preferably the modified starch is modified toinclude a hydroxy propyl group.

In a yet further embodiment the at least one starch layer comprises awater soluble polymer. Preferably, the starch layer comprises 1 to 20%by weight of a water soluble polymer, more preferably from 4 to 12% byweight of a water soluble polymer. Exemplary, but non-limiting, watersoluble polymers are selected from the group consisting ofpolyvinylacetate, polyvinyl alcohol or mixtures thereof. Polyvinylalcohol is a particularly preferred water soluble polymer.

In some embodiments the at least one starch layer may comprise water,preferably up to 20% by weight water, more preferably up to 12% byweight water. In some embodiments the water may serve as a plasticiser.

In some embodiments the moisture content of the at least one starchlayer is generally the equilibrium moisture content at the environmental% RH. For example, the equilibrium moisture content ranges from about 4%at low % RH to more than 15% at high % RH.

In a still yet further embodiment the at least one starch layercomprises one or more polyol plasticisers, preferably up to 20% byweight of one or more polyol plasticisers, more preferably up to 12% byweight of one or more polyol plasticisers. Exemplary, but non-limiting,polyol plasticisers are selected from the group consisting of sorbitol,glycerol, maltitol, xylitol, and mixtures thereof.

In another embodiment the at least one starch layer may also comprise upto 50% by weight of natural unmodified starch.

In some embodiments the at least one starch layer comprises mixtures ofstarches and/or modified starches, for example, mixtures of high and lowamylose starch, wherein one or more of the starch components may bemodified.

In a further embodiment the at least one starch layer comprises alubricant. Preferred lubricants are C₁₂₋₂₂ fatty acids and/or C₁₂₋₂₂fatty acid salts. Preferably, the C₁₂₋₂₂ fatty acid and/or a C₁₂₋₂₂fatty acid salt are present in an amount up to 5% by weight.

In some embodiments the at least one starch layer comprises one or morenanomaterials. Preferably, the nanomaterials are exfoliated within astarch nanocomposite. Exemplary nanomaterials include clays and modifiedclays particularly ‘hydrophobically modified layered silicate clays’.Preferred clays include montmorillonite, bentonite, beidelite, mica,hectorite, saponite, nontronite, sauconite, vermiculite, ledikite,magadite, kenyaite, stevensite, volkonskoite or a mixture thereof.

A ‘hydrophobically modified layered silicate clay’ or ‘hydrophobic clay’is preferably a clay modified by exchange with a surfactant comprisinglong chain alkyl groups such as a long chain alkylammonium ion, forexample, mono- or di-C₁₂-C₂₂ alkylammonium ion, wherein polarsubstituents such as hydroxyl or carboxyl are not attached to the longchain alkyl. Examples of suitable clays include CLOISITE® 20A orCLOISITE® 25A from Southern Clay Products Inc.

In some embodiments the starch layer and/or the other layer may comprisecolourants.

Adhesive

In some embodiments the at least one other layer may be fixed to the atleast one starch layer through use of a suitable adhesive. This assistsin minimising slip and therefore maintaining excellent barrierperformance. Numerous suitable adhesives would be readily apparent tothose having skill in the present art. Preferably the adhesive isselected so as to chemically bond to the at least one starch layer.Preferred adhesives comprise one or more polyurethanes.

Advantageously, the use of an adhesive overcomes or minimises the needfor modified or grafted other layers to be utilised as tie layers.Accordingly, for example, standard film polyethylene grades may besuccessfully employed as polyolefin other layers in the multi-layerfilm. This is desirable from a cost consideration.

Other suitable adhesives include EVA copolymers, acrylic copolymers andterpolymers, ionomers, metallocene derived polyethylene, ethyleneacrylic ester terpolymers and ethylene vinyl acetate terpolymers.

Those skilled in the art will be familiar with other adhesive laminationtechnology that would be suitable for adhering various types ofplastics, including heat activated and UV activated systems. Exemplaryadhesives include, polyurethane, epoxy, nylon, acrylic and acrylate.

Method of Preparation of the Multilayer Film

The multilayer film can be made by a variety of processes. Themultilayer film can be made by co-extrusion, coating, and otherlaminating processes. The film can also be made by casting or blown filmprocesses.

Coextrusion tends to use tie layers, and utilises modified other layers,such as modified (grafted) polyolefins. Coextrusion is generally able toachieve thinner overall gauges. Lamination is more suitable for thickermultilayer films utilising an adhesive.

In one embodiment a three layer film is provided comprising an innerstarch layer and two outer polyolefin layers. In other embodimentsadhesive layers may be employed between the starch layer and thepolyolefin layers thus yielding a five layer film.

Applications

It should be understood by those with skill in the art that a three orfive layer film is only one of many possible embodiments that employsstarch and other layers. The number of layers and their relativethicknesses may be adjusted depending on the function or end-use of thefilm.

Additionally, further film layers comprising other materials commonlyutilised in barrier film applications may be envisaged. Exemplaryfurther film layers include metallised films, non-polymer films and thelike.

The multilayer film has many uses including grocery bags, stretch-wraps,food packaging films, package containers, package lids where low watervapour and oxygen transmission rates are required.

Accordingly, in a further aspect there is provided a use of a multilayerfilm according to any of the aforementioned embodiments in packaging,preferably in packaging foodstuffs.

In a yet further aspect there is provided an article of manufacturecomprising the multilayer film according to any one of theaforementioned embodiments. A preferred article of manufacture is a foodpackage, such as containers, lids, bags, stretch-wraps and films.

Throughout this specification, use of the terms “comprises” or“comprising” or grammatical variations thereon shall be taken to specifythe presence of stated features, integers, steps or components but doesnot preclude the presence or addition of one or more other features,integers, steps, components or groups thereof not specificallymentioned.

DETAILED DESCRIPTION OF THE INVENTION

It will now be convenient to describe the invention with reference toparticular embodiments and examples. These embodiments and examples areillustrative only and should not be construed as limiting upon the scopeof the invention. It will be understood that variations upon thedescribed invention as would be apparent to the skilled addressee arewithin the scope of the invention. Similarly, the present invention iscapable of finding application in areas that are not explicitly recitedin this document and the fact that some applications are notspecifically described should not be considered as a limitation on theoverall applicability of the invention.

Polyolefins

Suitable LLDPE, HDPE and polypropylene can be produced by a Ziegler,single-site, or any other olefin polymerization catalyst. Zieglercatalysts and co-catalysts are well known in the art. Metallocenesingle-site catalysts are transition metal compounds that containcyclopentadienyl (Cp) or Cp derivative ligands. For example, U.S. Pat.No. 4,542,199, teaches the preparation of metallocene catalysts.Non-metallocene single-site catalysts containing heteroatomic ligands,e.g., boraaryl, pyrrolyl, azaborolinyl or quinolinyl are also well knownin the art.

The HDPE can also be multimodal. By “multimodal” it is meant that thepolymer comprises at least two components, one of which has a relativelylow molecular weight, the other a relatively high molecular weight. Themultimodal polyethylene can be produced by polymerization usingconditions that create a multimodal polymer product. This can beaccomplished by using a catalyst system with two or more differentcatalytic sites or by using two or multi-stage polymerization processeswith different process conditions in the different stages (e.g.different temperatures, pressures, polymerization media, hydrogenpartial pressures, etc). Multimodal HDPE may be produced by a multistageethylene polymerization, using a series of reactors, with comonomeraddition in only one of the reactors.

Modified Starch

A preferred modified starch component is hydroxypropylated amylosestarch. Other substituents may be hydroxyethyl or hydroxybutyl to formhydroxyether substitutions, or anhydrides such as maleic phthalic oroctenyl succinic anhydride can be used to produce ester derivatives. Thedegree of substitution (the average number of hydroxyl groups in a unitthat are substituted) is preferably 0.05 to 1.5. A preferred starch is ahigh amylose maize starch. Another preferred starch is a high amylosetapioca starch. A preferred modified starch component is ahydroxypropylated high amylose starch (for example ECOFILM® marketed byNational Starch and Chemical Company, or Gelose® A939 marketed byPenford).

The other starch component, if utilised, is any commercially availablestarch. This may be derived from, for example, wheat, maize, tapioca,potato, rice, oat, arrowroot, and pea sources. These starches may alsobe chemically modified.

Water Soluble Polymer

The water soluble polymer component of the starch layer is preferablycompatible with starch, water soluble, biodegradable and has a lowmelting point compatible with the processing temperatures for starch.Polyvinyl alcohol is a preferred polymer but polymers of ethylene-vinylalcohol, ethylene vinyl acetate or blends with polyvinyl alcohol may beused. A preferred concentration range 4 to 12% by weight, morepreferably 8%-12%.

Plasticiser

A range of plasticisers and humectants are useful additions to thestarch layer, in order to aid processing and control and stabilize themechanical properties of the barrier material, in particular in reducingdependency on moisture content and relative humidity. The desiredplasticiser content depends primarily on the required processingbehaviour during a (co)-extrusion process and subsequent blowing orstretching processes as well as on the required mechanical properties ofthe end product.

Cost and food contact are important issues in choosing the appropriateplasticizer. The preferred plasticizer is a mixture of polyols, inparticular sorbitol, and one or more other polyols particularlyglycerol, maltitol, mannitol and xylitol, although erythritol, ethyleneglycol and diethylene glycol are also suitable. The plasticizer plays atriple role:

-   -   1. it provides suitable rheology for the extrusion compounding        process and for the lamination process,    -   2. it positively affects the mechanical properties of the        product and,    -   3. it may act as an anti-retrogradation or anti-crystallizing        agent.

The preferred plasticizer content is up to 20% by weight of the starchlayer depending on the particular application and co-extrusion orlamination process.

Sorbitol, glycerol and maltitol blends are particularly suitable formodifying the mechanical properties of the formulation, as is xylitoland blends of xylitol with sorbitol and glycerol. The larger the numberof OH groups, the more effective the plasticiser is in reducingcrystallisation. Sorbitol, maltitol and xylitol are particularly goodhumectants. Glycerol helps dissolve polyvinylalcohol during processing.Crystallisation is observed when sorbitol is used on its own. Somepolyols (sorbitol and glycerol in particular) may exhibit migration tothe surface, where either an opaque crystalline film may form in thecase of sorbitol, or an oily film in the case of glycerol. Blendingvarious polyols inhibits this effect to varying degrees. Stabilisationmay be enhanced with the addition of glycerol monostearate and sodiumstearoyl lactylate as emulsifiers. Furthermore, synergistic effects withsalt result in stronger effects on mechanical properties.

Other Plasticizers

Polyethylene glycol compounds may be used as emulsifying agents,plasticizers or humectants. Polyethylene oxide and polyethylene glycolalternately or together may also provide an increased water resistance,to prevent swelling which may result in delamination in multi-layerstructures (MLS).

An alternative plasticiser is epoxidized linseed oil or epoxidizedsoybean oil. Being hydrophobic these additives may improve moisturesensitivity of the material. These plasticisers, preferably stablilizedwith an emulsifying system, aid processing but do not result in asignificant further reduction in Young's modulus. Other plasticizersmore commonly used in the PVC industry may be suitable, includingtributyl citrate, 2,2,4 trimethyl-1,3-pentanediol diisobutyrate, andacetyl tri-ethyl citrate.

One may use up to 20% of a humectant or water binding agent or gellingagent which may act as a (co)plasticiser such as carrageenan, xanthangum, gum arabic, guar gum or gelatine. Other humectants may be used suchas sugar or glucose. Biopolymers such as carrageenan, typically used infood products as thickeners and partially soluble in cold water, fullysoluble in hot water, are suitable to tailor mechanical properties. Bybinding water these components may have a significant plasticizingfunction. Gelatine may be added to improve the mechanical properties andreduce moisture sensitivity. Xanthan Gum has a high water holdingcapacity and also acts as an emulsifier and in starch compositions hasan anti-retrogradation effect. Gum Arabic may also be used as atexturiser and film former, and the hydrophilic carbohydrate andhydrophobic protein enable its hydrocolloid emulsification andstabilization properties. Guar gum has similar anticrystallisationeffects in starch compositions. Another suitable humectant is glyceryltriacetate.

Fatty Acid and/or Fatty Acid Salt

Fatty acids and/or fatty acid salts may be used as lubricants. Thestarch layer preferably comprises between 0.1 to 1.5% by weight of aC₁₂₋₂₂ fatty acid and/or a C₁₂₋₂₂ fatty acid salt. The fatty acid and/orfatty acid salt component is more preferably present in concentrationsof 0.6 to 1%. Stearic acid is a particularly preferred component. Sodiumand potassium salts of stearic acid may also be used. Cost can be afactor in the choice of this component but lauric, myristic, palmitic,linoleic and behenic acids are all suitable.

Adhesive

Polyurethane based adhesives are particularly suitable for fixing theother layer to the starch layer. The polyurethane adhesive may beprepared in situ through reaction of one or more isocyanates with thestarch layer. Through reaction of the surface hydroxyl functions of thestarch with isocyanate, urethane functions are formed. Preferredisocyanates are diisocyanates. Those skilled in the art would be able toselect suitable isocyanates from the wide range typically employed inthe art of polyurethane synthesis.

Alternatively, the polyurethane adhesive may comprise one or morepolyols. Such two component systems comprising diisocyanate and polyolare well known in the art.

The adhesives may or may not contain solvent. The solvent may be organicor water based.

Exemplary isocyanates include methylene diphenyl diisocyanate andtoluene diisocyanate. Exemplary polyols include polyether polyols suchas polyethylene glycol or polypropylene glycol and polyester polyolssuch as adipate based polyols.

EXAMPLES

OTR was measured using ASTM F 1927-98 and WVTR was measured using ASTM F1249-01. All component weights are expressed on a dry basis.

Example 1

A starch film was prepared by extrusion processing of a mixture of 88.5%by weight modified starch (ECOFILM®, National Starch and ChemicalCompany), 9% by weight polyvinylalcohol (Elvanol® 71-30), 2% by weightCloisite 20A (Southern Clay Products) and 0.5% stearic acid and castinginto a 300 μm sheet. This was then adhesively laminated on each side to100 μm HDPE film using MOR Free PU adhesive (Rohm and Haas). Thelamination was performed on a standard laminating machine.

Samples were conditioned for 2 weeks at 50% and 75% RH (for OTR) and 38°C./90% RH (for WVTR) and measured after equilibration.

Tables 1 and 2 collect the results.

TABLE 1 Nominal % of Oxygen Transmission Rate Specimen starch layer(cm³/m² · 24 h at 23° C., thickness Sample thickness 1 atm pure oxygen)(micron) PE/Starch/PE 60 50% RH <0.05 507 <0.05 470 75% RH <0.05 496<0.05 468

TABLE 2 Water Vapour Nominal % of Transmission Rate Specimen starchlayer (g/m² · 24 h at thickness Sample thickness 38° C., 90% RH)(micron) PE/Starch/PE 60 3.3 507 100/300/100 3.2 470

Examples 2 & 3

A starch film was prepared by extrusion processing of a mixture of 88.5%by weight modified starch (ECOFILM®, National Starch and ChemicalCompany), 9% by weight polyvinylalcohol (Elvanol® 71-30), 2% by weightCloisite 20A (Southern Clay Products) and 0.5% stearic acid and castinginto a 150 urn sheet. This was then adhesively laminated on each side to50 μm (Example 2) or 35 μm (Example 3) HDPE using a polyurethaneadhesive system from Specialty Adhesives and Coatings. The laminationwas performed on a standard laminating machine.

Samples were conditioned for 2 weeks at 50% and 75% RH (for OTR) and 38°C./90% RH (for WVTR) and measured after equilibration.

Table 3 collects the results.

TABLE 3 Nominal % Oxygen Transmission Rate Water Vapour TransmissionRate of starch (cm³/m² · 24 h) (g/m² · 24 h at 38° C., 90% RH) layer 50%RH, Thickness 75% RH, Thickness Thickness Sample thickness 23° C.(micron) 23° C. (micron) WVTR (micron) Example 2 60 0.55 258 0.91 2653.00 260 PE/Starch/PE 0.46 262 0.98 254 3.22 255 Example 3 68 0.51 2221.03 228 5.16 225 PE/Starch/PE 0.55 227 1.16 225 5.30 225

Example 4

A starch film was prepared by extrusion processing of a mixture of 90.5%by weight modified starch (ECOFILM®, National Starch and ChemicalCompany), 9% by weight polyvinylalcohol (Elvanol® 71-30) and 0.5% byweight stearic acid and casting into a 350 μm sheet. This was thenadhesively laminated on each side to 50 μm HDPE using a polyurethaneadhesive system from Specialty Adhesives and Coatings. The laminationwas performed on a standard laminating machine.

Samples were conditioned for 2 weeks at 50% and 75% RH (for OTR) and 38°C./90% RH (for WVTR) and measured after equilibration.

Table 4 collects the results.

TABLE 4 Nominal % Oxygen Transmission Rate Water Vapour TransmissionRate of starch cm³/m² · 24 h g/m² · 24 h at 38° C., 90% RH layer 50% RH,Thickness 75% RH, Thickness Thickness Sample thickness 23° C. (micron)23° C. (micron) WVTR (micron) PE/Starch/PE 78 0.05 465 0.15 465 3.25 4720.05 468 0.16 455 3.10 468

Example 5

A starch film was prepared by extrusion processing of a mixture of 90.5%by weight modified starch (ECOFILM®, National Starch and ChemicalCompany), 9% by weight polyvinylalcohol (Elvanol® 71-30) and 0.5% byweight stearic acid and casting into a 350 μm sheet. This was thenadhesively laminated to 50 μm HDPE on one side, and an 80 μmpolypropylene film on the other side using a polyurethane adhesivesystem from Specialty Adhesives and Coatings. The lamination wasperformed on a standard laminating machine.

Samples were conditioned for 2 weeks at 50% and 75% RH (for OTR) and 38°C./90% RH (for WVTR) and measured after equilibration.

The results are collected in Table 5.

TABLE 5 Nominal % Oxygen Transmission Rate Water Vapour TransmissionRate of starch cm³/m² · 24 h g/m² · 24 h at 38° C., 90% RH layer 50% RH,Thickness 75% RH, Thickness Thickness Sample thickness 23° C. (micron)23° C. (micron) WVTR (micron) PE/Starch/PP 73 PP side <0.05 484 0.11 4952.21 498 facing <0.05 500 0.11 500 2.16 490 permeant PE side Notmeasured 0.10 500 Not measured facing 0.16 484 permeant

Comparative Example 1

A starch film was prepared by extrusion processing of a mixture of 88.5%by weight modified starch (ECOFILM®, National Starch and ChemicalCompany), 9% by weight polyvinylalcohol (Elvanol® 71-30), 2% by weightCloisite 20A (Southern Clay Products) and 0.5% stearic acid and castinginto a 290 μm sheet.

Samples were conditioned for 2 weeks at 50% and 75% RH, and OTR measuredafter equilibration. The results are collected in Table 6.

TABLE 6 Oxygen Transmission Rate Specimen cm³/m² · 24 h at 23° C.,thickness Sample 1 atm pure oxygen micron Starch Sheet 50% RH 0.21 2830.21 289 75% RH 1.48 282 1.30 285

Comparative Example 2

A starch film was prepared by extrusion processing of 100% by weightmodified starch (ECOFILM®, National Starch and Chemical Company), andcasting into a 300 μm sheet.

Samples were conditioned for 2 weeks at 50% and 75% RH (for OTR) and 38°C./90% RH (for WVTR) and measured after equilibration.

Table 7 shows the results.

TABLE 7 Oxygen Transmission Rate Water Vapour Transmission Rate cm³/m² ·24 h g/m² · 24 h at 38° C., 90% RH 50% RH, Thickness 75% RH, ThicknessThickness Sample 23° C. (micron) 23° C. (micron) WVTR (micron) StarchSheet 0.50 295 1.30 260 337 290 0.49 320 1.26 295 374 275

SUMMARY OF EXAMPLES

Table 8 collects the OTR and OPV (oxygen permeation value) for each ofthe Examples. The OPV are normalised to 1 mm thick samples, based on thecore starch layer thickness only.

TABLE 8 Core Skin OTR (50% RH) OPV (50% RH) OTR (75% RH) OPV (75% RH)Thickness Thickness [cm³/m² · [cm³ · mm/m² · [cm³/m² · [cm³ · mm/m² ·Example micron micron 24 h] 24 h · atm] 24 h] 24 h · atm] 1 300 100<0.05 <0.05 <0.05 <0.05 2 150 50 0.51 0.08 0.95 0.14 3 150 35 0.53 0.081.10 0.16 4 350 50 0.05 0.02 0.16 0.06 5 350 50 <0.05 <0.05 0.13 0.05CE1 290 0 0.21 0.06 1.40 0.41 CE2 300 0 0.50 0.15 1.30 0.40It is evident from the results that the multilayer films of Examples 1to 5 show excellent barrier performance. It is noted that where the corestarch layer is approximately 300 micron thick, OTR is substantiallyreduced in samples having outer layers, relative the performance of astarch layer alone. Thinner starch core layers have low OTR at high(75%) RH relative to starch alone. Starch layers alone, in the absenceof outer layers, indicate very high WVTR.

1. A multilayer film comprising: (a) at least one starch layercomprising a modified starch; and (b) at least one other layer having awater vapour permeability coefficient less than 1 g·mm/m²·24 hr·atmmeasured at 38° C. and 90% relative humidity; and wherein the totalthickness of the at least one starch layer is greater than 20% of thetotal thickness of the multilayer film and wherein the modified starchhas a degree of substitution less than 1.5.
 2. The multilayer filmaccording to claim 1 wherein the water vapour permeability coefficientof the at least one other layer is less than 0.5 g·mm/m²·24 hr·atm,preferably less than 0.2 g·mm/m²·24 hr·atm.
 3. The multilayer filmaccording to claim 1 wherein the total thickness of the at least onestarch layer is greater than 30% of the total thickness of themultilayer film, preferably greater than 40% of the total thickness ofthe multilayer film, more preferably greater than 50% of the totalthickness of the multilayer film.
 4. The multilayer film according toclaim 1 wherein the at least one other layer comprises a polyolefin,polyethylene terephthalate, nylon, polyvinylchloride and polyvinylidenedichloride or mixtures thereof.
 5. The multilayer film according toclaim 4 wherein the polyolefin is selected from the group consisting ofethylene homopolymers, propylene homopolymers, interpolymers of ethyleneand propylene and interpolymers of ethylene or propylene with one ormore C₄-C₁₀ α-olefins, cyclic olefin polymers and copolymers, biaxiallyorientated polypropylene, chemically modified polyolefins and mixturesthereof.
 6. The multilayer film according to claim 4 wherein thepolyolefin is selected from the group consisting of high densitypolyethylene, low density polyethylene, linear low density polyethylene,polypropylene, biaxially orientated polypropylene and mixtures thereof.7. The multilayer film according to claim 1 comprising an inner starchlayer and two outer polyolefin layers.
 8. The multilayer film accordingto claim 1 wherein the at least one starch layer is fixed to the atleast one other layer by an adhesive.
 9. The multilayer film accordingto claim 8 wherein the adhesive comprises a polyurethane and/orpolyurethane urea.
 10. The multilayer film according to claim 1 whereinthe film has an oxygen permeability coefficient (OPC) less than 0.6 cm³mm/m²·24 h·atm at 50% RH, preferably less than 0.3 cm³ mm/m²·24 h·atm at50% RH, more preferably less than 0.1 cm³ mm/m²·24 h·atm at 50% RH, mostpreferably less than 0.05 cm³ mm/m²·24 h·atm at 50% RH.
 11. Themultilayer film according to claim 1 wherein the total thickness of themultilayer film is between 10 and 1000 micron.
 12. The multilayer filmaccording to claim 1 wherein the at least one starch layer has a totalthickness between 100 and 600 micron and the at least one other layerhas a total thickness between 10 and 400 micron.
 13. The multilayer filmaccording to claim 1 wherein the at least one starch layer has a totalthickness between 10 and 60 micron and the at least one other layer hasa total thickness between 5 and 40 micron.
 14. The multilayer filmaccording to claim 1 wherein the modified starch comprises a highamylose starch.
 15. The multilayer film according to claim 1 wherein themodified starch is chemically modified so as to replace hydroxylfunctionality with functionality selected from the group consisting ofethers and esters and mixtures thereof.
 16. The multilayer filmaccording to claim 1 wherein the modified starch is chemically modifiedto include a hydroxyalkyl C₂₋₆ group or modified by reaction with ananhydride of a carboxylic acid.
 17. The multilayer film according toclaim 1 wherein the at least one starch layer further comprises at leastone water soluble polymer.
 18. The multilayer film according to claim 17wherein the at least one water soluble polymer is selected from thegroup consisting of polyvinyl alcohol and polyvinyl acetate and mixturesthereof.
 19. The multilayer film according to claim 1 wherein the atleast one starch layer further comprises at least one plasticiser. 20.The multilayer film according to claim 19 wherein the at least oneplasticiser comprises one or more polyols.
 21. A method of making themultilayer film of claim 1 wherein the method comprises at least one ofthe steps of co-extrusion, coating, casting or film blowing. 22.(canceled)
 23. An article of manufacture comprising the multilayer filmaccording to claim
 1. 24. The article according to claim 23 wherein thearticle is a package.